Throughout this process and since we first called for a full market investigation in 2011, we have been open and fully supportive of the detailed and thoughtful investigative work undertaken by the CMA and we will continue to provide all necessary and relevant information as we move towards the final report in December.

We will now examine today's publications in detail, along with the analysis that underpins them.

We will also continue to work constructively with the CMA as this process continues to help ensure that the opportunity presented by this investigation is fully grasped, and that the final result is an enduring outcome that gives customers confidence, allows regulators to regulate, and encourages investors to invest in the Great Britain energy market.

There are millions of customers paying too much for their energy bills - but they don't have to.

Whilst competition is delivering benefits to increasing numbers of customers, mainly through the growth of smaller suppliers with cheaper fixed-price deals, the majority of us are still on more expensive default tariffs.

The confusing way energy is measured and billed can make comparing deals understandably daunting.

The result is that some energy suppliers know they don't have to work hard to keep these customers.

Energy bills have been 5% higher than they should have been Credit: PA

Customers have been paying about 5% a year -or £60 - too much for gas and electricity, the Competition and Markets Authority has confirmed.

"Lack of awareness of what deals are available, confusing and inaccurate bills and the real and perceived difficulties of changing suppliers all deter switching - and the higher price levels reflect that suppliers can charge higher prices to these disengaged customers," the CMA said in its report.

Its year-long inquiry into the big six energy suppliers focused on prices between 2009 and 2013.

It outlined plans to encourage customers to switch, but stopped short of recommending a break-up of the energy giants, saying competition in wholesale markets was working well.